Using isalpha function with string pointers - c++

Question

Hey I'm quite new to programming and I'm having trouble using the isalpha function in my programme. This a part of the code for a palindrome class. What I'm trying to do is remove all the non alphabetic characters from the input. So if the user inputs "Hi, How are you" I need to first count the size of the array of just the letters then in my removeNonLetters subclass, I need to get rid of the non alphabetical characters. Can someone please help me with this. Thank you so much!
#include <iostream>
#include <string>
#include <stdio.h>
#include <algorithm>
#include <cctype>
#include <cstring>
#include <ctype.h>
using namespace std;
class palindrome
{
private:
int only_letters_size;
string input_phrase;
string* only_letters;
public:
string inputPhrase();
string removeNonLetters();
string* new_Array;
int size_new_Array;
};
string palindrome::inputPhrase()
{
cout << "Input phrase: "; //asks the user for the input
getline(cin,input_phrase);
size_new_Array = input_phrase.length(); //creating a dynamic array to store
the input phrase
new_Array = new string[size_new_Array];
int i;
for (i=0; i<size_new_Array; i++)
{
new_Array[i]=input_phrase[i];
}
only_letters_size = 0;
while(new_Array[i])
{
if (isalpha(new_Array[i])) //PROBLEM OCCURS HERE
{
only_letters_size=only_letters_size+1;
}
}
cout << only_letters_size << endl;
return new_Array;
}
string palindrome::removeNonLetters()
{
int j=0;
int str_length = new_Array.length(); //string length
only_letters = new string[only_letters_size];
for (int i=0;i<size_new_Array;i++) //PROBLEM OCCURS HERE AS WELL
{
if (isalpha(new_Array[i]))//a command that checks for characters
{
only_letters[j] = new_Array[i];//word without non alphabetical c
characters is stored to new variable
j++;
}
}
cout << only_letters << endl;
return only_letters;
}

Answer 1

I've found the best way to determine if a string is a palindrome is to walk toward the center from both sides. In your case I would just opt to skip non-alpha characters like so.
bool is_palindrome(string mystring)
{
int start = 0, end = mystring.length() - 1;
while (start < end)
{
// Skip over non-alpha characters
while (!isalpha(mystring[start]))
{
start++;
}
while (!isalpha(mystring[end]))
{
end--;
}
if (tolower(mystring[start]) != tolower(mystring[end]))
{
return false;
}
else
{
start++;
end--;
}
}
return true;
}
If you must save the input first and remove nonalpha characters, I would do it like this.
string remove_non_alpha(string mystring)
{
string ret_string = "";
for (int i = 0; i < mystring.length(); i++)
{
if (isalpha(mystring[i]))
{
ret_string += tolower(mystring[i]);
}
}
return ret_string;
}
And then feed the result into the above function.

Answer 2

Sorry for being hard, but your trying far too much copying around. You can achieve all this with one single loop after retrieving your data and all on one single string object (unless you want to keep the original input for some other purposes):
getline(cin,input_phrase);
std::string::iterator pos = input_phrase.begin();
for(char c : input_phrase)
{
if(isalpha(c))
{
*pos++ = tolower(c);
}
}
input_phrase.erase(pos, input_phrase.end());
After that, your string is ready to use...
Explanation:
std::string::iterator pos = input_phrase.begin();
An iterator something similar than a pointer to the internal data of the string. We keep the position to move the alpha only characters to, skipping the non-alpha ones.
for(char c : input_phrase)
Simply iterating over all characters...
if(isalpha(c))
The essential check, is the current character an alpha one?
*pos++ = tolower(c);
If so, convert it to lower case immediately. Assign it to the current string position, and advance the "pointer" (iterator!).
input_phrase.erase(pos, input_phrase.end());
And at very last, drop the remaining part of the string occupied with surplus characters. You might note that there might be some characters you wanted to keep within, but you copied these to a position more to the left already...